The present invention is directed to contrast agents for biomedical use comprising aqueous colloidal dispersions. More specifically, the present invention is directed to liquid in liquid emulsions in which the dispersed liquid undergoes a temperature or pressure activated phase shift from a dispersed liquid to a dispersed gaseous form which is efficient in reflecting ultrasound energy in a manner which is diagnostically useful.
Various contrast agents for use with diagnostic ultrasound, including echocardiography, have been described. A review of the subject is found in Ophir and Parker, Ultrasound in Med. and Biol. (1989), 15:319-333. The acoustic backscatter arising from these agents, the property typically associated with the contrast effect, can be attributed to unique properties which they possess as solids, liquids or gases. While solids and liquids reflect sound to a similar degree, gases are known to be more efficient and are the preferred media for the development of ultrasound contrast agents.
Known liquid agents for ultrasound include emulsions and aqueous solutions. About these the authors of the above review stated, xe2x80x9cthe idea of using liquid emulsions of certain lipids in aqueous vehicles was tested by Fink et al. (1985). Unfortunately, no enhancement of backscatter was observable in these experiments.xe2x80x9d
Known solid agents include collagen microspheres. However, the poor acoustic backscatter of the solid-liquid interface prevents their wide spread use.
Known gaseous agents include microbubbles stabilized by the addition of various amphiphilic materials to the aqueous media, by materials that increase viscosity, and gaseous precursors, either. as solid particles or liposomes. However, the liposomes can only contain water soluble gases and are thus limited in the stability of the microbubbles they can form, since one of the characteristic physical properties of many of the chemicals which form especially stable microbubbles is immiscibility in water. The solid particles must be reconstituted immediately before use, requiring extensive preparation, and must be used quickly, since the microbubbles disappear soon after the particles have completely dissolved. My own prior U.S. patent application Ser. No. 07/761,311 is directed to methods of determining the relative usefulness of gases as ultrasound contrast agents, and identifies particularly useful gases for that purpose.
One study has been identified which used the injection of a liquid which boils at a temperature below the boiling point of the organism under study to enhance the ultrasound Doppler signal (Ziskin MC, Bonakdarpour A, Weinstein D P, Lynch P R: Contrast Agents For Diagnostic Ultrasound. Investigative Radiology 7:500-505, 1972). In this study a number of solutions or liquids were injected intraarterially into dogs and the Doppler signal detected five cm below the injection site. This study reported that, xe2x80x9cether, which produced the greatest contrast effect of any agent that we tried, is a liquid which boils vigorously at body temperature and therefore acts as a very active source of bubbles.xe2x80x9d The report further stated that xe2x80x9cether, however, is a toxic substance when injected in large amounts. Injections of 20 mL proved fatal in our experiments.xe2x80x9d This paper does not discuss methods of stabilizing any materials suitable for later use as ultrasound agents. Non-colloidal ether is too toxic for intravenous administration, where the greatest need for a useful contrast agent exists.
The biocompatability of emulsions which include fluorocarbons is a serious safety concern. For example, Clark et al. (Clark L C, Becattini F, Kaplan S: Can fluorocarbon emulsions be used as artificial blood? Triangle 11:115-122, 1972) state, in speaking about the choice of fluorocarbon, xe2x80x9ctheir vapor pressures range from zero to about 640 torr. Those with vapor pressures over 400 torr, of course, cannot be used because they would boil when infused in the. blood stream.xe2x80x9d Later in the same article they state, xe2x80x9cIf a fluorocarbon with a vapor pressure of over 50 torr is given intravenously, death results in a few hours, and when the chest is opened, the lungs do not collapse.xe2x80x9d The same author, L. C. Clark, reports a similar conclusion exactly twenty years later, xe2x80x9cIf practical methods cannot be found to prevent or counteract HNCL (hyperinflated non-collapsible lungs), and if HNCL occurs in other species, then only fluorocarbons boiling above 150xc2x0 C. can be considered safe,xe2x80x9d Clark C L, Hoffmann R E, Davis S L: Response of the rabbit lung as a criterion of safety for fluorocarbon breathing and blood substitutes, Biomat., Art. Cells and Immob. Biotech., 20:1085-1099, 1992.
The stability of liquid-liquid emulsions presents another problem. A body of knowledge surrounds the stability of emulsions and the ability to predict stability from solubility; this theory is called the Ostwald ripening theory (Kabalnov A S, Shchukin E D; Ostwald Ripening Theory: Applications To Fluorocarbon Emulsion Stability, Advances in Colloid and Interface Science, 38:69-97, 1992). This paper states, simply, that the more soluble is the dispersed phase liquid of an emulsion in the continuous phase, the less stable is the emulsion. These same authors tested the stability of a dodecafluoropentane emulsion at 25xc2x0 C. (Kabalnov A S, Makarov K N, Shcherbakova O V: Solubility of fluorocarbons in water as a key parameter determining fluorocarbon emulsion stability. J Fluorine, Chemistry 50:271-284, 1990). They determined that their emulsion had an Ostwald ripening rate of 1.4xc3x9710xe2x88x9218 cm3/s. Converting this rate constant into useful terms shows that Kabalnow et al""s dodecafluoropentane emulsion, which had an initial size of 211 nm, would experience a particle mean diameter growth rate of 11 nm/sec or 660 nm/minute. At this rate of particle growth, such an emulsion would have a shelf life of less than a minute, and therefore be unworkable as a commercial product.
Thus, there is a need for an effective ultrasound contrast composition with extended shelf life, which is relatively easy to manufacture, and which is biocompatible and convenient to use.
In order to meet these needs, the present invention is directed to stable colloidal dispersions of the liquid-in-liquid type. The colloids are composed of a liquid dispersed phase which has a boiling point below the body temperature of the organism on which an ultrasound contrast study is desired, typically about 37-40xc2x0 C. These emulsions are preferably composed of a dispersed phase liquid which has a boiling point between xe2x88x9220 and 37xc2x0 C.
Preferably the liquid dispersed phase is selected from the group of chemicals consisting of aliphatic hydrocarbons, organic halides or ethers, or combinations thereof, which have six or fewer carbon atoms and an upper limit of molecular weight of about 300. Among organic halides, the fluorine-containing chemicals are preferred, since they form stable emulsions and are relatively non-toxic. Especially preferred are n-pentane, isopentane, neopentane, cyclopentane, butane, cyclobutane, decafluorobutane, dodecafluoropentane, dodecafluoroneopentane, perfluorocyclopentane and mixtures thereof. Preferably, the colloidal dispersion contains the dispersed phase at a concentration of 0.05 to 5.0% w/v. Optimally, the concentration range is 0.5 to 3.5% w/v.
The colloidal dispersion can be stabilized by the addition of various amphiphilic materials, including anionic, nonionic, cationic, and zwitterionic surfactants, which typically lower the interfacial tension between the dispersed liquid and water to below 26 dynes/cm. optimally, these materials are nonionic, synthetic surfactant mixtures, containing a fluorine-containing surfactant, such as the Zonyl brand series and a polyoxypropylene-polyoxyethylene glycol nonionic block copolymer.
The liquid continuous phase of the colloidal dispersion comprises an aqueous medium. This medium can contain various additives to assist in stabilizing the dispersed phase or in rendering the formulation biocompatible. Acceptable additives include acidifying agents, alkalizing agents, antimicrobial preservatives, antioxidants, buffering agents, chelating agents, suspending and/or viscosity-increasing agents, including triodobenzene derivatives, such as iohexol or iopamidol, and tonicity agents. Preferably, agents to control the pH, tonicity, and increase viscosity are included. Optimally, a tonicity of at least 250 mOsm is achieved with an agent which also increases viscosity, such as sorbitol or sucrose.
The colloidal dispersions are typically formed by comminuting a suspension of the dispersed phase in the continuous phase by the application of mechanical, manual, or acoustic energy. Condensation of the dispersed phase into the continuous phase is also acceptable. The preferred mode is to use high pressure comminution.
The invention relates to agents that enhance the contrast in an ultrasound image generated for use in medical and veterinary diagnosis. These agents are comprised of biocompatible colloidal dispersions in which the dispersed phase is a liquid under the conditions of the manufacturing process and which undergoes a phase shift to become a dispersed gas or kugelschaum at or about the time of administration to the organism under study.
In order to provide a clear and consistent understanding of the present invention and claims, including the scope given to such terms, the following definitions relating to the invention are provided:
Colloidal Dispersion: A system having at least one substance as a liquid or gas (the dispersed phase) which is immiscible and finely divided and distributed evenly throughout at least one second substance which forms the dispersion medium or continuous liquid phase.
Biocompatible: Capable of performing functions within or upon a living organism in an acceptable manner, without undue toxicity or physiological or pharmacological effects.
Liquid: The state of matter in which a substance or substances exhibit(s) a characteristic readiness to flow, little or no tendency to disperse, and relatively high incompressibility.
Gas: The state of matter of a substance or substances which is distinguished from the solid or liquid states by very low density and viscosity, relatively great expansion and contraction with changes in temperature and pressure, and the spontaneous tendency to become distributed uniformly throughout any container.
Phase Shift: A change of state between liquid and gas due to changes in temperature and/or pressure.
Kugelschaum: One of the two forms of foams in the classification of Manegold (Manegold, E. xe2x80x9cSchaum, Strassenbau, Chemie und technik.xe2x80x9d Heidelberg, 1953, which is incorporated herein by reference). Specifically, the kugelschaum or spherical foam, consists of widely separated spherical bubbles and is distinct from the polyederschaum or polyhedral foams, which consist of bubbles that are nearly polyhedral in shape, having narrow lamellar films of very low curvature separating the dispersed phase.
Low Boiling Liquid: A liquid with a boiling point, under standard pressure conditions, below 40xc2x0 C. Low boiling liquids useful in the invention include, but are not limited to, hydrocarbons, organic halides, and ethers, where, in any case, the molecule has 6 carbon atoms or less.
Aliphatic Hydrocarbons: The group of alkane, alkene, alkyne, cycloalkane, and cycloalkene organic compounds. Of these, only compounds having boiling points below about 40xc2x0 C. (such as those having six or fewer carbon atoms) and which are thus capable of undergoing a liquid to gas phase transition after administration to a subject from part of this invention. Aliphatic hydrocarbons useful in the invention include, but are not limited to, those selected from the chemical group: Isobutane; Isobutylene; 1-Butene; 1,3-Butadiene; n-Butane; 2-Butene {trans}; 2-Butene {cis}; Vinyl acetylene; 1-Butyne; Neopentane; Butadiyne; 1,2-Butadiene; Cyclobutane; 1-Butene, 3-methyl; Cyclopropane, 1,1-dimethyl; 1,3-Dioxolane-2-one, 4-methyl; 3-Butene-2-one, 4-phenyl {trans}; 1,5-Heptadiyne; 1,4-Pentadiene; 2-Butyne; Butane, 2-methyl; Cyclopropane, 1,2-dimethyl {trans, dl}; 1-Butyne, 3-methyl; 1-Pentene; 1-Butene, 2-methyl; 1,3-Butadiene, 2-methyl; 1-Butene-3-yne, 2-methyl; Isoprene; Cyclopropane, ethyl; n-Pentane; Cyclobutane, methyl; 2-Pentene {trans}; 2-Pentene {cis}; Cyclopropane, 1,2-dimethyl {cis}; and 1-Nonene-3-yne.
Organic Halides: The group of compounds containing at least one carbon or sulfur atom and at least one halogen atom, i.e., chlorine, bromine, fluorine, or iodine. Of these, only the members of the group having boiling points below about 40xc2x0 C. (such as those with six or fewer carbon atoms) which are capable of undergoing a phase transition upon administration to an organism with a body temperature of up to 40xc2x0 C. form part of the invention. Examples of such organic halides include: Methane, tetrafluoro; Methane, chlorotrifluoro; Ethane, hexafluoro; Ethane, perfluoro; Methane, fluoro; Ethylene, tetrafluoro; Sulfur hexafluoride; Methane, bromotrifluoro; Methane, difluoro; and like compounds.
Ethers: The class of organic compounds in which two hydrocarbon groups or derivatives thereof are linked by an oxygen atom. For the purposes of the present invention the following are examples of some, but not necessarily all, ethers which can be used: methyl ether, ethyl methyl ether, methyl vinyl ether, methyl isopropyl ether, 1,2-epoxypropyl ether, diethyl ether, ethyl vinyl ether, and vinyl ether.
Fluorine-Containing Compounds: A compound containing at least one fluorine atom. Some useful fluorine-containing compounds are listed above as organic halides. See also the examples below.
The colloidal dispersions of the invention can be emulsions or microemulsions.
Emulsion: A colloidal dispersion of one immiscible liquid dispersed in another liquid in the form of droplets, whose diameter, in general, are between 100 and 3000 nm and which is typically optically opaque, unless the dispersed and continuous phases are refractive index matched. Such systems possess a limited stability, generally defined by the application or relevant reference system, which may be enhanced by the addition of amphiphilic materials or viscosity enhancers.
Microemulsion: A stable liquid monophasic and optically isotropic colloidal dispersion of water and water-immiscible liquids stabilized by amphiphilic materials in which the dispersions have appreciable light scattering properties (meaning they can appear optically clear or milky but are reddish or yellowish if observed by transmitted light) and the diameters of the particles are, in general, between 5 and approximately 140 nm.
In a preferred embodiment of the present invention, the colloidal dispersion contains one or more amphiphilic materials to improve the stability of the formulation.
Amphiphilic Material: A substance which is strongly adsorbed at an interface and which normally produces a dramatic reduction in the interfacial tension with small changes in the bulk phase concentration. Examples include synthetic surfactants, naturally occurring materials such as biocompatible proteins, lipids, sterols, alginates, cellulose derivatives, and finely divided organic or inorganic particulate solids.
Organic Particulate Solids: include sugars, proteins, amino acids, lipids, nucleic acids, and others.
Inorganic Particulate Solids: include aluminas, carbonates, bicarbonates, silicates, aluminasilicates, phosphates, and others.
Interface: The region or boundary of the physical world that lies between two distinct and identifiable phases of matter, herein limited to liquid-liquid, liquid-solid, solid-gas, and liquid-gas.
Interfacial Tension: The force per length which exists at the interface between two distinct and identifiable phases of matter.
Stability: The time lapse from initial preparation and packaging during which a colloidal dispersion continues to fulfill all chemical and physical specifications with respect to identity, strength, quality, and purity which have been established according to the principles of Good Manufacturing Practice, as set forth by appropriate governmental regulatory bodies.
Surfactants: The group of amphiphilic materials which are manufactured by chemical processes or purified from natural sources or processes. These can be anionic, cationic, nonionic, and zwitterionic, as are well known in the art. Such materials are described in Emulsions: Theory and Practice, Paul Becher, Robert E. Krieger Publishing, Malabar, Fla., 1965 which is incorporated by reference herein.
The continuous phase of the colloidal dispersion of the present invention is an aqueous medium.
Agueous Medium: A water-containing liquid which can contain pharmaceutically acceptable additives such as acidifying agents, alkalizing agents, antimicrobial preservatives, antioxidants, buffering agents, chelating agents, complexing agents, solubilizing agents, humectants, solvents, suspending and/or viscosity-increasing agents, tonicity agents, wetting agents or other biocompatible materials. A tabulation of ingredients listed by the above categories, can be found in the U.S. Pharmacopeia National Formulary, 1990, pp. 1857-1859, which is incorporated herein by reference.
A preferred embodiment of the present invention includes the use of at least one amphiphilic material from the groups consisting of biocompatible proteins, fluorine-containing surfactants, polyoxypropylene-polyoxyethylene glycol nonionic block copolymers, and surfactants.
Polyoxypropylene-Polyoxyethylene Glycol Nonionic Block Copolymers: The surfactants which are available from BASF Performance Chemicals, Parsippany, N.J. under the trade name PLURONIC(copyright) are polyoxyalkylene ethers of high molecular weight having water soluble, surface active and wetting properties and which consists of the group of surfactants designated by the CTFA name of poloxamer 108, 188, 217, 237, 238, 288, 338, 407, 101, 105, 122, 123, 124, 181, 182, 183, 184, 212, 231, 282, 331, 401, 402, 185, 215, 234, 235, 284, 333, 334, 335, and 403.
Fluorine-Containing Surfactant: A surfactant containing one or more fluorine molecules. Some but not necessarily all fluorine containing surfactants, useful in this invention can be selected from the group consisting of: telomer B containing fluorinated surfactants available from Du Pont, Wilmington, Del. under the trade name of ZONYL(copyright) which are fluorochemical surface active agents (including ZONYL(copyright) FSA, FSP, FSE, UR, FSJ, FSN, ESO, FSC, FSK, and TBS), the fluorochemical surfactants from 3M Industrial Chemical Products Division St. Paul, Minn. under the trade name of FLUORAD(copyright) which are surface acting agents or surfactants (including FC-95, FC-98, FC-143, FC-170C, FC-171, FC-430, FC-99, FC-100, FC-120, FC-129, FC-135, FC-431, FC-740), the perfluoroalkylpoly(oxyethylene) surfactants described by Mathis et al. (J Am Chem Soc 106, 6162-6171 (1984), incorporated herein by reference), the fluoroalkylthio-etherpoly(oxyethylene) surfactants described by Serratrice et al. (J Chim Phys 87, 1969-1980 (1990), incorporated herein by reference), the perfluoroalkylated polyhydroxylated surfactants of Zarif et al. (J Am Oil Chem Soc 66, 1515-1523 (1989), incorporated herein by reference), the fluorosurfactants available from Atochem North America, Philadelphia, Pa. under the trade name of Forafac.
Biocompatible Proteins: The group of proteins, regardless of source and whether obtained by extraction of animal, plant, or microbiological tissue or obtained from recombinant biotechnology, which is capable of performing its function of stabilizing the colloidal dispersions of the instant invention in an acceptable manner, without undue toxicity or physiological or pharmacological effects. Some acceptable biocompatible proteins can be selected from the group consisting of albumin, alpha-1-antitrypsin, alpha fetoprotein, aminotransferases, amylase, C-reactive protein, carcinoembryonic antigen, ceruloplasmin, complement, creatine phosphokinase, ferritin, fibrinogen, fibrin, transpeptidase, gastrin, serum globulins, hemoglobin, myoglobin, immunoglobulins, lactate dehydrogenase, lipase, lipoproteins, acid phosphatase, alkaline phosphatase, alpha-1-serumprotein fraction, alpha-2 serum protein fraction, beta protein fraction, gamma protein fraction, gamma-glutamyl transferase, and other proteins.
A preferred process for manufacturing the colloidal dispersions of this disclosure is comminution. An alternative process for manufacturing is condensation.
Comminution: The process of forming a colloidal dispersion by mixing the liquid dispersed and continuous phases together and then causing a decrease in size of the particles of the dispersed phase from large particles to the size required, using mechanical energy generated by mixing manually, mechanically, or by the action of ultrasound. Appropriate mixing can be achieved in a Microfluidic""s Model 110 Microfluidizer apparatus, as described in U.S. Pat. No. 4,533,254, incorporated herein by reference. An acceptable alternative is the Rannie High Pressure Laboratory Homogeniser, Model Mini-Lab, type 8.30H, or equivalent.
Condensation: The process of forming a colloidal dispersion by starting with the dispersed phase as a gas, placing it in contact with the liquid continuous phase and then causing an increase in size of the particles of the dispersed phase from a molecular ensemble to the size required, generally by inducing a phase change of the dispersed gas to a liquid by the action of changes in the system temperature, pressure, or both.